TLDR Summary: In spite of the grossly misleading framing given by study author Dr Frederick Rivara, a long-time helmet advocate, the latest head injury study from Seattle shows total injury rates and head injury rates drop dramatically in cities that implement bike share programs, compared against control cities without bike share.

Nursing Professor Janessa Graves’ look at head injury rates published in the American Journal of Public Health earlier this week has received significant publicity both in the mainstream media and among those who seem to think cycling is a dangerous activity.

The University of Washington Health Science Center in Seattle announced Graves’ research in science by press release. The results show a significant increase in the percentage of head injuries among bike riders:

Researchers looked at the change in head injuries before and after the implementation of bike-share programs in five cities in the United States and Canada. They also gathered similar data for five cities that did not have bike-share programs.

Of all bicycle-related injuries that occurred in bike-share cities during the study period, the proportion that were head injuries rose from 42 percent to 50 percent after bike-share program implementation. No such increase was found in cities without these programs.

“Our results suggest that bike-share programs should place greater importance on providing helmets so riders can reap the health benefits of cycling without putting themselves at greater risk for injury,” said Janessa Graves, affiliate faculty at the Harborview Injury Prevention and Research Center and assistant professor at the Washington State University College of Nursing.

“It doesn’t take much effort to wear a helmet when you bike,” Graves said, “but doing so could make all the difference.”

The study authors reach a conclusion that makes sense given this dramatic increase in head injuries:

Results suggest that steps should be taken to make helmets available with PBSPs. Helmet availability should be incorporated into PBSP planning and funding, not considered an afterthought following implementation.

The numbers as presented in the press release, in the media, and in the abstract certainly seem alarming and support the helmet advocacy promoted by the study authors. What happens when we look at the actual numbers?

In spite of public funding of this research by the National Institutes of Health, the entire study remains behind a commercial paywall. Others with access to the paper, however, have teased out the numbers. Here’s what they’ve found.

For bike share cities in the two years before implementation of bike share, the researchers counted an average of 438 non-head injuries and 319 head injuries each year. This adds up to the 42% of injuries are head injuries as reported in the WaPo, NPR, and in the press release.

For the year following implementation of bike share, those numbers are 272 for non-head injuries and 273 head injuries.

So yeah, head injuries now make up 50% injuries, but everybody has missed the real story: A DRAMATIC DECLINE IN BOTH HEAD INJURIES AND TOTAL INJURIES after bike share programs have been implemented.

Furthermore, the number of “moderate to severe” head injuries also dropped in these cities by 27%, from 162 per year to 119 per year.

Here’s the data as graphed from the study data that includes the control cities. You’ll see the control cities changed very little, while the total injuries, head injuries and moderate-to-severe head injuries all dropped after bike share began.

Given these results, the conclusion that “steps should be taken to make helmets available” for bike share systems doesn’t follow. Maybe helmets would have reduced the total number of head injuries, but this study doesn’t illustrate that at all.

Helmets prevent leg injuries!

This study was authored by Dr. Frederick Rivara, Adjunct Professor of Epidemiology at the University of Washington School of Public Health in Seattle. Rivara still brags about his infamously flawed “seminal research on the effectiveness of bicycle helmets” in 1989 that has been thoroughly discredited. This 1989 landmark study compared inner city children riding with heavy traffic in commercial and industrial areas, who didn’t wear helmets, with the path-riding children of wealthier suburbanites who rode with helmets. Rivara and his colleague Dr Robert Thompson concluded that helmets reduce head injury rates by 85%. Thompson and Rivara steadfastly stood by their 85% conclusion even after others took the same data to show that helmets reduced the rate of leg injuries by a similar amount.

The cities with bike-share programs were Boston, Miami Beach, Minneapolis, Washington, D.C., and Montreal, Quebec. None of the programs provided helmets to users during the study period. The control cities without bike-share programs were Los Angeles, Milwaukee, New York City, Seattle and Vancouver, British Columbia.

After I wrote this I found Streetsblog covers this same headline grabbing news, taking the Washington Post to task for their misleading characterization of the study. Angie Schmitt spoke with University of British Columbia public health professor Kay Ann Teschke about the results, who says “I really feel badly that this wrong information is getting out there. I don’t really think the results support their conclusion much.”

Thanks Lars. The Thompson & Rivara 1989 study where we get the mythical and widely repeated 85% figure was also authored by Dr Rivara, who authored this latest bike share study. I’ve expanded the paragraph about Rivara a little to further explain his participation.

As far as I can tell, Dr Rivara’s research is mostly funded through either government grants from agencies such as the NIH, and sometimes through the non-profit Snell Memorial Foundation.

He’s certainly capable of quality research and as an epidemiologist he understands what he’s doing. I’d argue that this specific bike share study is valuable. I think I’d conclude that we should take a look at why the number of head injuries declined less than the number of non-head injuries. Drawing the conclusion he did is a real puzzle.

The Snell Memorial Foundation may be non-profit, but that doesn’t mean that they are somehow without a bias. SMF’s sole purpose is to test and certify helmets for a variety of human-powered activities and motorsports; the bulk of their income is from “program revenues,” which is presumably testing fees paid by helmet manufacturers. They are undoubtedly interested in promoting the use of helmets.

I’m not sure where the graph above came from, but I don’t think it accurately shows the data in the paper. If you look at the data, there was a similar drop in number of head injuries in the control cities as in the bike-share cities–i.e., ride-share did NOT decrease the total number of head injuries injuries. Here’s what they list:

Head injuries in ride-share cities
Before ride-share: 638
After ride-share: 273

Head injuries in control (no ride-share) cities
Before ride-share started in other cities: 712
After ride-share started in other cities: 342

The decreases were 57% in ride-share cities and 52% in non ride-share cities. I did a statistical test on the numbers (Fisher’s exact test) and the result wasn’t statistically significant (p=.242. –“Significant” is usually defined as a p<.05.)

In both ride-share and non ride-share cities, there also were drops in non-head injuries.

Non-head injuries in control (no ride-share) cities
Before ride-share started in other cities: 1151
After ride-share started in other cities: 611

In ride-share cities, there was a 69% drop in non-head injuries after implementation of ride-share. In non ride-share cities, there was only a 47% reduction. This difference is statistically significant (p=.0001, Fisher's exact test). That's quite interesting, though I can't think of a reason why it would happen.

As nearly as I can tell, the authors' conclusions (as far as they go) are consistent with their data. However, it would've been REALLY nice if they'd been able to divide up their data by helmet use and whether or not the injured were ride-share riders. If they're correct, most of the injuries would've been among unhelmeted ride-share riders.

The good news is that riding appears to be getting safer everywhere, but even more so in ride-share cities. CAVEAT: I'm not a statistician–I'm a biologist–and may have screwed up the numbers or statistical tests. I'm happy to be corrected.

Mea culpa. I contacted Dr. Teschke, who created the graph above. She gently pointed out that I had neglected to notice that the pre-implementation data covered 2 years, rather than 1, and thus needed to be divided by 2 to make the pre- and post-implementation periods comparable. Thus her graph represents the data correctly. Unfortunately, that doesn’t mean that riding is getting safer everywhere. Only in ride-share cities.

Uh…no. The title says there was a dramatic drop in head injuries when cities implement bike-share programs. There was a significant drop in *all injuries* and a significant drop in *non-head* injuries. However, I don’t see any significant change in head injuries. For bike share cities, there were 319 head injuries per year pre-implementation and 273 post-implementation. For control cities, there were 356 pre- and 342 post-implementation. Using a Fisher’s exact test, there’s no significant difference. Or am I missing something…again?!?